Funds are requested for a MDS Nordian Inc. Gammacell-40 Exactor Self-contained Low Dose Rate Research animal (rodent) irradiator to support the ongoing NIH-funded individual-initiated research programs in a new state of the art animal facility located within a recently erected research building (Hill Pavilion) on the campus of the University of Pennsylvania School of Veterinary Medicine. Ten core laboratories comprise the list of the major users of the requested instrument. Each will have laboratories in either the Hill Pavilion or the adjacent research laboratories at the Penn Vet School and all of them will house their mice and perform ongoing experiments requiring whole animal irradiation and extended maintenance of irradiated animals in ongoing experiments in the animal facility in the Hill Pavilion. These core laboratories will constitute the majority (>70%) of the usage of the irradiator by the School of Veterinary Medicine. There is a group of additional Veterinary School faculty that will utilize this instrument on an infrequent (<5%) basis. In addition, since the vivarium in Hill will be shared with the School of Medicine, there will be substantial usage by NIH-funded faculty from University of Pennsylvania School of Medicine who will occupy part of the new animal facility in the Hill Pavilion (25%). A specific need for this instrument arises because the new animal facility in which the animals of the core user group are housed is a closed facility in which animals taken to other buildings for irradiation cannot be returned to the investigator's designed area. As a consequence, it is impossible to perform long-term experiments requiring maintaining the animals beyond a single day after irradiation, or that require comparison of irradiated and non-irradiated experimental groups. As detailed in the project descriptions, these include irradiated, auto-reconstituting models, bone marrow chimeras, induction of immunodeficiency, among others. The University of Pennsylvania School of Veterinary Medicine has already made a significant financial commitment to support the procurement of this instrument, by constructing the room in which the irradiator will be housed. In addition, the School has committed funds for the ongoing service contract to maintain and service the instrument, and will provide salary support for a supervisor. The University will provide support for security. The installation of the Gammacell-40 in the Hill Pavilion vivarium will make possible studies that will elucidate immune responses to pathogens and tumors, as well as studies of stem cells. Each of these will advance our understanding of human diseases. [unreadable] [unreadable] [unreadable]